One idea could be an OC with the Rinnegan being a transfer student to the Highschool DxD world. Their Rinnegan abilities could attract the attention of different factions. For example, they might be able to use the Preta Path to absorb the demonic energy attacks of the devils, which would both intrigue and alarm the devils in the school.
A plot could revolve around the OC's Rinnegan being the key to an ancient prophecy in the Highschool DxD universe. This could lead to the OC being hunted by some forces who want to prevent the prophecy from coming true, while being protected by others who believe it will bring about a new era of peace or power. Maybe the OC has to learn to control the Rinnegan's powers quickly to survive.
Perhaps the OC with the Rinnegan discovers a hidden connection between the world of Highschool DxD and the Naruto universe from which the Rinnegan originates. This could involve finding a way to use the Rinnegan to travel between the two worlds or unlock secrets that were long forgotten. As a result, the OC becomes a sort of bridge between the two very different universes, causing all kinds of chaos and new alliances to form.
An OC (Original Character) in a Highschool DxD fanfiction having the Rinnegan could be an extremely powerful and unique addition. The Rinnegan, from the Naruto universe, brings a whole set of abilities like the Six Paths powers. In the context of Highschool DxD, this OC could use these powers in various ways. For example, the ability to see chakra could be translated into seeing the magical energy in the DxD world. They could use the gravitational powers of the Rinnegan to manipulate the environment during battles against devils, angels, or fallen angels. It would make for a very interesting and overpowered character in the Highschool DxD fanfiction setting.
One idea could be an OC gunslinger who is actually a reincarnated warrior from a different dimension. They could have a special gun that has the power to nullify the abilities of devils, angels, and fallen angels. This would create a unique dynamic as they try to find their place in the Highschool DxD world while being hunted by different factions who either want to use their power or eliminate them.
A plot could involve the mute OC being a new transfer student in the school. Their muteness makes them an object of curiosity and some characters might initially misjudge them.
The OC's Rinnegan in a Highschool DxD fanfiction would be extremely powerful. It could give the character abilities like seeing through illusions, which would be very useful against devils known for their trickery. The power to summon creatures from the Six Paths could also be a game - changer. It could summon powerful beings to fight alongside the OC in battles against other supernatural entities in the DxD world.
A good plot idea could be that the OC Dragon God has to hide their true identity in the high school. They are there to find a certain magical item that is hidden in the school. As they search, they get involved with the main characters and have to deal with the normal high school drama as well as the supernatural threats that come their way.
The balance can also be achieved by making the OC need to train and develop their Rinnegan powers gradually. They start with basic abilities and as the story progresses, they learn more complex and powerful ones. This way, they face challenges appropriate to their current power level at each stage of the story, and the power growth feels more natural and not overly sudden.
One idea could be an OC who is a dragon slayer but has a unique power that sets them apart. For example, they can absorb the powers of the dragons they slay instead of just killing them. This could lead to all kinds of new storylines as they become more powerful and are hunted by both good and bad forces in the Highschool DxD universe.
A plot could revolve around the Sharingan OC being a target because their Sharingan is seen as a powerful tool. Devils might want to use it for their own gain, and angels might see it as a threat. The OC then has to fight to protect themselves and their friends. Also, the OC could use their Sharingan to unlock a hidden power within Issei, which changes the power balance in the DxD world. Another idea is that the OC comes from a parallel world where the Sharingan and DxD powers are combined, and they are on a mission to bring balance to the DxD world.
Maybe the OC is a descendant of an ancient race that was once closely associated with the power of Divine Dividing. They come to the high school to find out more about their heritage and end up getting involved in the power struggles between the different factions in the DxD world. This could involve making alliances and enemies along the way.
One idea could be an OC who is a transfer student with a unique power, like the ability to manipulate time slightly. This catches the attention of the main characters in the Highschool DxD world. The harem could start to form as different characters are intrigued by the OC's power and mysterious background.